profiles: Generate GHC's package database cache.

* guix/profiles.scm (ghc-package-cache-file): New procedure.
  (profile-derivation): Add 'ghc-package-cache?' keyword argument.  If true
  (the default), add the result of 'ghc-package-cache-file' to 'inputs'.
* guix/scripts/package.scm (guix-package)[process-actions]: Pass
  #:ghc-package-cache? to 'profile-generation'.
* tests/packages.scm ("--search-paths with pattern"): Likewise.
* tests/profiles.scm ("profile-derivation"): Likewise.

build-system/haskell: Update configure flags, 'haddock' and %standard-phases.

* guix/build/haskell-build-system.scm (%standard-phases): move 'haddock phase
  before 'install phase.
* guix/build/haskell-build-system.scm (haddock): Simplify it as the 'install
  phase takes care of copying files.
* guix/build/haskell-build-system.scm (configure): Add '--libsubdir' flag.
  Fix use of '--extra-include-dirs' and '--extra-lib-dirs' flags.  Use 'doc',
  'bin' and 'lib' outputs if they are defined.
* guix/build/haskell-build-system.scm (make-ghc-package-database, register):
  Aligh location of 'package.conf.d' directory with '--libsubdir' flag.

import: Add hackage importer.

* guix/import/hackage.scm: New file.
* tests/hackage.scm: New file.

import: Add hackage importer.

* guix/scripts/import.scm (importers): Add hackage.
* guix/scripts/import/hackage.scm: New file.
* po/guix/POTFILES.in: Add guix/scripts/import.scm.
* doc/guix.texi: Add section on 'hackage' importer.

gnu: Add libsecret.

* gnu/packages/gnome.scm (libsecret): New variable.

build: Enable silent rules by default.

* configure.ac: Use 'AM_SILENT_RULES'.

gnu: guix: Update development snapshot.

* gnu/packages/package-management.scm (guix-devel): Update to 0b13161.

tests: Skip 'guix-package-net.sh' when prerequisites aren't met.

* tests/guix-package-net.sh: Reinstate networking and shebang test that
  inadvertently vanished in commit 57b8623.

gnu: inetutils: Apply syslogd patch.

* gnu/packages/patches/inetutils-syslogd.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/admin.scm (inetutils)[source]: Use it.
  [native-inputs]: New field.

gnu: julia: Pass MARCH make flag.

* gnu/packages/julia.scm (julia)[arguments]: Pass "MARCH" flag for supported
  architectures.

gnu: julia: Mark as unsupported on ARM and MIPS.

* gnu/packages/julia.scm (julia): Add 'supported-systems' field.

gnu: wavpack: Fix wavpack.pc by passing '--libdir' to `configure'.

* gnu/packages/audio.scm (wavpack)[arguments]: New field.

doc: Refer to "Substitutes" from "Setting Up the Daemon".

Suggested by Mark H Weaver.

* doc/guix.texi (Setting Up the Daemon): Add xref to "Substitutes".

doc: Mention the relation between module names and file names.

* doc/guix.texi (Package Modules): Mention the relation between module
  names and file names.

doc: Mention that the current generation is not deleted.

* doc/guix.texi (Invoking guix package): Mention that the current
  generation is not deleted.  This is a followup to d26eb84.

gnu: kbd: Update to 2.0.2.

* gnu/packages/linux.scm (kbd): Update to 2.0.2.

gnu: Add python-drmaa.

* gnu/packages/python.scm (python-drmaa, python2-drmaa): New variables.

gnu: Add Solfege.

* gnu/packages/music.scm (solfege): New variable.

gnu: Add Csound.

* gnu/packages/audio.scm (csound): New variable.

gnu: Add libiec61883.

* gnu/packages/linux.scm (libiec61883): New variable.